Fine gold alloy wire for bonding of a semi-conductor device

ABSTRACT

A fine gold alloy wire of high tensile strength for bonding semiconductor elements is disclosed. The wire consists essentially of 0.0003 to 0.010 wt % of at least one rare earth element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y, the balance being Au and incidental impurities. The wire does not present a deformed loop and has greater bond strength if it contains 0.0003 to 0.010 wt % of at least one rare earth element of the Cerium Group selected from the group consisting of La, Ce, Pr, Nd and Sm and 0.0001 to 0.0060 wt % of at least one element selected from among Ge, Be and Ca.

This application is a continuation of Ser. No. 825,942 filed Feb. 3,1986 (abandoned); which is a continuation of Ser. No. 586,642 filed Mar.7, 1984 (abandoned); which is a CIP of Ser. No. 419,840 filed Sept. 20,1982 (abandoned).

TECHNICAL FIELD

The present invention relates to a fine gold alloy wire having hightensile strength both at room temperature and high temperatures andwhich is suitable for use in wire-bonding a semiconductor device to alead frame.

BACKGROUND ART

In fabricating semiconductor devices such as transistors, ICs and LSIs,electrodes on a chip are connected to external leads by gold wires.Semiconductor devices are typically fabricated by the following steps:

(a) prepare a lead material from a strip of Cu or Cu alloy or Ni or Nialloy having a thickness of 0.1 to 0.3 mm;

(b) stamp out a lead frame conforming to the shape of the semiconductordevice to be fabricated;

(c) apply high-purity Si or Ge semiconductor elements to selected areasof the lead frame by thermocompression with an electrically conductiveresin such as Ag paste or through a plating of Au, Ag, Ni or their alloyformed on one surface of the lead material;

(d) connect the semiconductor elements to the lead frame by gold wires(this is a bonding step);

(e) enclose with a protective plastic package the semiconductorelements, gold wires and parts of the lead frame to which thesemiconductor elements have been bonded;

(f) remove unnecessary parts from the lead frame to form discrete leads;and

(g) apply a soldering material to the legs of the leads to enableconnection of the semiconductor device to the substrate.

In the bonding step (d), the gold wire is fixed sequentially at theproper points of the semiconductor elements and the lead frame (kept at150°-300° C.) by a manual or automatic bonding machine. That is, first,the gold wire is heated at its tip with an oxy-hydrogen flame or byelectrical means to let it take the form of a ball, which is pressedagainst the semiconductor, then an extension of the gold wire is pressedatainst a point on the lead frame to be fixed thereon and the wire iscut to finish the bonding cycle. Then, this cut end becomes the tipserving as a point to be pressed against the next bonding point on thesemiconductor in the subsequent bonding cycle.

As bonding at higher speed and more highly integrated circuits aredesired, the use of finer and stronger gold wires is necessary. But thecurrently used wires are made of pure gold which has a relative lowtensile strength at room and high temperatures and cannot be drawn to asmaller diameter of 0.05 mm or less without accompanying frequentbreakage of the wire, and even if pure gold could be drawn into wiresthat fine, they would often break during the bonding step. What is more,because of the low softening point of pure gold, the crystal grains ofthe wire being cut with a flame or by electrical means recrystallize tobecome bigger and brittle, and when the gold ball is pressed at between150° and 300° C., the bonded wire softens to deform the wire loopconnecting the semiconductor elements and lead frame and may causeshorting. The pure gold wire also does not have satisfactory bondstrength with respect to the semiconductor elements and lead frame.

Gold bonding wires should have the highest possible content of gold inorder to make most use of its physical, electrical and chemicalproperties. However, wires with a gold content of 99.999% or more havinga diameter of 50 μm or less have a tensile strength of only 6 to 7 g atroom temperature and have a still lower value at elevated temperatures.

Therefore, the primary object of the present invention is to provide agold wire which (1) has a very high gold content and (2) is sufficientlyprotected against increased brittleness and deterioration in otherproperties, and which (3) claims tensile strength values at roomtemperature and elevated temperatures which are at least 50% higher thanthose previously attainable even when the wire is drawn to a diameter of50 μm or less.

DISCLOSURE OF THE INVENTION

The present invention eliminates the problems of the conventional puregold bonding wire and provides (1) a gold alloy wire having high tensilestrength at either room temperature or high temperature that can bedrawn to a diameter as small as 0.05 mm or less and which can be used asa bonding wire without breaking, and (2) a gold alloy wire that provideshigh bond strength and has a sufficiently high softening point toprevent the formation of brittle coarse crystal grains due torecrystallization and the presence of a deformed loop. The gold alloywire (1) is characterized by a composition consisting essentially of0.0003 to 0.010 wt% of at least one rare earth element selected from thegroup consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu, Sc and Y, the balance being Au and incidental impurities. The goldalloy wire (2) is characterized by a composition consisting essentiallyof 0.0003 to 0.010 wt% of at least one rare earth element of the CeriumGroup selected from the group consisting of La, Ce, Pr, Nd and Sm,0.0001 to 0.0060 wt% of at least one element selected from among Ge, Beand Ca, the balance being Au and incidental impurities.

The criticality of the components of each composition is describedbelow.

(a) Rare earth elements

The rare earth elements are equivalent to one another in their abilityto increase the tensile strength of the wire at either room temperatureor high temperature. If their content is less than 0.0003 wt%, thedesired tensile strength at room and high temperatures is not achieved,and if their content exceeds 0.010 wt%, the wire becomes brittle andcannot be smoothly drawn to a smaller size. Therefore, in the presentinvention, the content of the rare earth elements is between 0.0003 and0.010 wt%.

Generally, as more rare earth metal is added to the gold wire, itstensile strength for both room temperature and elevated temperatures isincreased, and at the same time, its hardness is increased. Wires whichare too hard may damage the chip. If the amount of the rare earth metalis excessively great, the gold wire becomes less elastic and forms aloop which is not high enough to prevent contact with the chip edge.Rare earth metals used as dopants are generally highly oxidizable, andif they are used in amounts which are too large, an oxide film forms toprevent strong bonding to contact points on the IC chip. According tothe finding of the present inventors, these problems can be eliminatedby using not more than 0.01% of a rare earth metal, and this is why theupper limit of a rare earth metal used in the present invention is0.01%.

In order to meet the two requirements that the gold content be as highas possible and that the desired value of tensile strength be ensuredboth at room temperature and elevated temperatures, the preferred rangeof the content of the rare earth metal is between 0.0003% and less than0.0010%.

(b) Ge, Be and Ga

These elements are equivalent to one another in that when combined withrare earth elements of cerium group, they achieve the followingadvantages: they increase the softening point of the wire to therebyprevent it from becoming brittle during the bonding step and prevent thepresence of a deformed loop; they provide increased bond strength; andthey further enhance the tensile strength of the wire at room and hightemperatures. If the content of these elements is less than 0.0001 wt%,their intended effects are not achieved, and if their content exceeds0.0060 wt%, the wire becomes brittle and cannot be drawn withouttroubles to a smaller size, and what is more, rupture easily occurs atthe grain boundary when the wire is heated to the bonding temperature.Therefore, in the present invention, the content of Ge, Be and Ca islimited to the range of from 0.0001 to 0.0060 wt%.

German Patent Publication No. 1608161 published in September 1970discloses a gold wire having a rare earth metal added to gold. However,in the specific embodiment shown, Ce-Mischmetal is incorporated in anamount of 0.015%, and claim 1 states that a rare earth metal isincorporated in an amount of 0.001 to 0.1% (0.005-0.05% in claim 2).Therefore, the gold content of the wire shown in the embodiment which issupposed to illustrate the best example is not more than 99.985%, andthe broadest possible range of the gold content is from 99.999 to 99.9%(claim 1). This shows that for the purposes envisioned in German PatentPublication No. 1608161, the minimum required gold content is 99.9%, andfor obtaining the best result, the gold content must be about 99.985%,i.e. the content of rare earth metals is a little less than 0.015%. Inother words, the German Patent explicitly shows that a rare earth metalused in an amount of less than 0.001% is entirely ineffective forachieving the purposes of the invention.

If only one example of an invention is shown in the specification, thatis supposed to be the best mode of embodiment. The specification ofGerman Patent Publication No. 1608161 shows that 0.015% of Ce-Mischmetalis used in the best mode of embodiment, but by using this amount ofCe-Mischmetal, the gold content of the wire is reduced to 99.985%, andthe specification says nothing about the defects which may occur inconsequence of this reduction. Apparently, the inventors of the Germanpatent did not recognize at all the importance of maintaining the goldcontent as high as possible. A wire whose gold content is 99.985% may bestronger than a wire containing 99.99% or more gold. However, there isno doubt that the former type of gold wire loses at least one of theproperties we expect from a wire having a very high gold content.

German Patent Publication No. 1608161 gives no answer to the followingquestion: Even if the amount of a rare earth metal is limited to notmore than 0.001% in order to maintain the gold content of a wire, willthe resulting wire have a significantly improved strength? claim 2 ofthis German Patent states that the preferred amount of a rare earthmetal to be added to gold ranges from 0.005 to 0.05%, and in the bestmode of embodiment, a gold wire consisting of 99.985% of gold and 0.015%of Ce-Meschmetal is shown. This reveals the entire lack of therecognition on the part on the inventors of the German Patent that ifthe content of a rare earth metal exceeded 0.01%, embrittlement andother defects that are described earlier in this specification wouldoccur.

In short, German Patent Publication No. 1608161 neither shows norsuggests that a gold wire having unexpectedly good properties can beprovided by incorporating a trace amount of a rare earth metal whilemaintaining the gold content of the wire at sufficiently high level.This is the basic idea of the present invention and is by no meansobvious from the cited publication. This conclusion may well bejustified by the fact that the gold wire shown in German PatentPublication No. 1608161 has not been commercialized although it becameknown to the public in 1970.

Japanese Patent Laid-Open Nos. 105968/78, 112060/78, 13740/81, 19629/81,49534/81 and 49535/81 show a wire for bonding a semiconductor devicewhich is made of Au and Ca optionally together with Be and Ge. However,none of these references show a gold wire incorporating a rare earthmetal. cl EXAMPLES

Gold alloy melts having the compositions indicated in Table 1 wereprepared by the conventional melting method. The melts were cast intoingots, which were rolled with a conventional grooved roll mill anddrawn to fine wire samples Nos. 1 to 120 of the present invention havinga diameter of 0.025 mm. The wire samples were subjected to a roomtemperature tensile test and a high temperature tensile test using theconditions that simulated the bonding step (temperature: 250° C., dwelltime: 30 seconds). The results of each test are indicated in Table 1.Samples Nos. 57 to 120 were used in actual bonding of semiconductorelements to a lead frame, and the bond strength and the presence of adeformed wire loop were checked. The results are also indicated inTable 1. The same tests were conducted on a pure gold wire and theresults are shown in Table 1 for the purpose of comparison.

    TABLE 1-1       tensile test tensile test  at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- Wire                    + strength gation strength     gation Samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc Y Ge Be Ca     impurities (g) (%) (g) (%)       Au alloy wires of the present invention 1 40      bal. 9.2 4 8.4 2 2     430      " 13.8 4 11.9 2 3 970      " 19.1 4 16.8 2 4  60     " 10.1 4     8.9 2 5  310     " 12.9 4 11.2 2 6  990     "19.5 4 17.1 2 7   50    "     9.4 4 8.5 2 8   490    " 14.2 4 12.1 2 9   950    " 18.8 4 15.9 2 10     40   " 9.3 4 8.2 2 11    440   " 13.8 4 11.9 2 12    940   " 18.3 4 15.1     2 13     40  " 9.2 4 8.1 2 14     610  " 14.1 4 12.0 2

    TABLE 1-2       tensile test tensile test  at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                    Au  breaking     elon- breaking elon- Wire                    + strength gation strength     gation Samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc Y Ge Be Ca     impurities (g) (%) (g) (%)       Au alloy wires of the present invention 15  950       bal. 18.1 4 15.4 2      16   50      " 9.6 4 8.3 2 17   520      " 13.7 4 11.5 2 18   940     " 17.6 4 15.2 2 19    50     " 9.0 4 7.9 2 20    440     " 13.2 4 11.3 2     21    950     " 16.8 4 14.7 2 22     40    " 9.3 4 8.1 2 23     480    "     13.9 4 11.5 2 24     960    " 17.4 4 14.9 2 25      50   " 9.0 4 7.9 2     26      520   " 13.6 4 11.4 2 27      950   " 17.1 4 14.3 2 28       60     " 9.5 4 8.3 2

    TABLE 1-3       tensile test tensile test  at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- Wire                    + strength gation strength     gation Samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc Y Ge Be Ca     impurites (g) (%) (g) (%)       Au alloy wires of the present invention 29  330      bal. 12.9 4 10.6     2 30  970      " 18.3 4 15.8 2 31   40     " 9.1 4 8.0 2 32   610     "     14.1 4 12.3 2 33   950     " 17.5 4 14.8 2 34    40    " 8.9 4 7.7 2 35       560    " 13.9 4 11.9 2 36    960    " 17.3 4 14.6 2 37     40   " 9.3     4 8.1 2 38     240   " 12.1 4 10.3 2 39     970   " 18.6 4 15.9 2 40      40  " 8.9 4 7.6 2 41      490  " 14.3 4 12.5 2 42      960  " 17.9 4     15.4 2

    TABLE 1-4       tensile test tensile test  at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- Wire                    + strength gation strength     gation Samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc Y Ge Be Ca     impurites (g) (%) (g) (%)       Au alloy wires of the present invention 43               50   bal. 9.5 4      8.2 2 44               610   "  14.5 4 13.1 2 45               980   "     18.9 4 16.2 2 46                40  " 8.4 4 7.3 2 47                730     " 13.7 4 11.3 2 48                940  " 16.2 4 13.8 2 49 10 20           " 9.6 4 8.2 2 50 90  100               " 11.5 4 9.3 2 51    100     30 40 30  50        " 12.8 4 10.2 2 52          100   70 40 30   " 13.1     3 10.4 2 53  50 40  30    30   40 60 50    " 14.4 4 12.4 2 54   480  300     210            " 19.8 4 17.1 2 55 470   50    220 190  30 20      " 19.4     4 17.0 2 56  470 190            110 230  " 18.7 4 16.4 2

    TABLE 1-5       tensile test tensile test   at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- bond  Wire                    + strength gation     strength gation strength deformed Samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho     Er Tm Yb Lu Sc Y Ge Be Ca impurites (g) (%) (g) (%) (g) loop       Au alloy wires of the present invention 57 32   95   bal. 9.9 4 8.9 2     7.0 none 58 525   87   " 14.5 4 12.5 2 8.0 " 59 979   85   " 19.8 4 17.3     2 9.0 " 60 513   12   " 14.1 4 12.1 2 7.5 " 61 520   195   " 14.3 4 12.2     2 8.5 " 62 503    13  " 14.3 4 12.2 2 7.5 " 63 486    102  " 14.6 4 12.7     2 8.0 " 64 501    199  " 15.1 4 13.0 2 8.5 " 65 491     11 " 14.3 4 12.2     2 7.5 " 66 513     98 " 14.8 4 12.9 2 8.5 " 67 509     196 " 15.1 4 13.2     2 9.5 " 68  33  101   " 10.6 4 9.3 2 7.0 " 69  428  98   " 13.6 4 11.8 2     8.0 " 70  964  97   " 19.9 4 17.5 2 9.0 "

    TABLE 1-6       tensile test tensile test   at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- bond  Wire                    + strength gation     strength gation strength deformed samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho     Er Tm Yb Lu Sc Y Ge Be Ca impurities (g) (%) (g) (%) (g) loop       Au alloy wires of the present invention 71  503   13   bal. 13.1 4     11.3 2 7.5 none 72  496   186   " 13.9 4 12.0 2 8.5 " 73  486    12  "     13.2 4 11.4 2 7.5 " 74  511    99  " 13.7 4 11.8 2 8.0 " 75  591    191     " 14.0 4 12.0 2 9.0 " 76  479     12 " 13.3 4 11.5 2 7.5 " 77  513     103 " 13.9 4 12.0 2 8.5 " 78  499     194 " 14.1 4 12.1 2 9.5 " 79   31     89   " 9.9 4 8.9 2 7.0 " 80   529  87   " 14.6 4 12.4 2 8.0 " 81   94     96   " 19.6 4 16.7 2 9.0 " 82   511  13   " 13.9 4 12.0 2 7.5 " 83   513      194   " 14.5 4 12.5 2 9.0 "

    TABLE 1-7       tensile test tensile test   at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- bond  Wire                    + strength gation     strength gation strength deformed samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho     Er Tm Yb Lu Sc Y Ge Be Ca impurities (g) (%) (g) (%) (g) loop       Au alloy wires of the present invention 84  496    12  bal. 13.9 4     12.0 2 7.5 none 85  503    106  " 14.3 4 12.3 2 8.0 " 86  516    187  "     14.7 4 12.6 2 8.5 " 87  509     11 " 14.0 4 12.1 2 7.5 " 88  505     112     " 14.5 4 12.4 2 8.5 " 89  515     194 " 14.8 4 12.7 2 9.5 " 90   33  86      " 9.9 4 8.7 2 7.5 " 91   495  89   " 14.5 4 12.4 2 8.0 " 92   975  98     " 18.6 4 15.9 2 9.0 " 93   486  12   " 14.2 4 12.1 2 7.5 " 94   465  195       " 14.7 4 12.6 2 8.5 " 95   473   12  " 14.3 4 12.1 2 7.5 " 96   456     103  " 14.6 4 12.4 2 8.0 "

    TABLE 1-8       tensile test tensile test  at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- bond  Wire                    + strength gation     strength gation strength deformed samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho     Er Tm Yb Lu Sc Y Ge Be Ca impurities (g) (%) (g) (%) (g) loop       Au alloy wires of the present invention 97  492    189  bal. 14.8 4     12.6 2 9.0 none 98  507     13 " 14.3 4 12.1 2 7.5 " 99  504     110 "     14.7 4 12.4 2 8.5 " 100  512     192 " 14.9 4 12.7 2 9.5 " 101   35  105       " 9.8 4 8.6 2 7.0 " 102   489  96   " 14.5 4 12.4 2 8.0 " 103   990     103   " 18.7 4 15.9 2 9.0 " 104   487  15   " 14.3 4 12.1 2 7.5 " 105     476  187   " 14.8 4 12.5 2 9.0 " 106   455   11  " 14.2 4 12.0 2 7.5 "     107   468   95  " 14.6 4 12.4 2 7.5 " 108   471   195  " 14.9 4 12.5 2     8.5 " 109   456    16 " 14.3 4 12.1 2 7.5 "

    TABLE 1-9       tensile test tensile test   at room at high Composition (wt %, ×     10.sup.-5) temperatures temperatures                     Au breaking     elon- breaking elon- bond  Wire                    + strength gation     strength gation strength deformed samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho     Er Tm Yb Lu Sc Y Ge Be Ca impurities (g) (%) (g) (%) (g) loop       Au alloy wires of the present invention 110     473    97 bal. 14.6 4     12.5 2 8.5 none 111     479    187 " 14.9 4 12.6 2 9.5 " 112 315      84     64  " 14.3 4 12.2 2 8.5 " 113 225 314       156 " 14.8 4 12.4 2 9.0 "     114   613  115   87 51 " 18.3 4 15.7 2 9.0 " 115 236  213  341  67 69 58     " 19.0 4 16.3 2 9.5 " 116  335     56  49 " 14.4 4 12.2 2 8.5 " 117  305      152   83 45  " 14.2 4 12.3 2 8.5 " 118 103      71  29 " 12.1 4 10.0 2     8.5 " 119 112 96 87  99  36 31 27 " 14.3 4 12.3 2 9.5 " 120 28 76 32 45     65  78 38 46 " 14.1 4 12.1 2 9.5 " pure AU wire           6.5 4.0 3.0     5.9 1.9 present

The data in Table 1 shows that the gold alloy wire samples of thepresent invention have greater tensile strength at both room and hightemperatures than the pure gold wire, and those having Ge, Be or Ca havethe additional advantage of significantly increased bond strength andare entirely free from a deformed loop. For these advantages, the goldalloy wire of the present invention can be drawn to a smaller diameterof 0.05 mm or less and can be used for bonding purposes withoutbreaking. By inclusion of Ge, Be or Ca, the softening point of the wireis further increased to eliminate the chance of embrittlement (due tothe formation of coarse grains) or occurrence of a deformed loop andachieve a high bond strength between semiconductor elements and leadframe. For these advantages, the present invention is very useful inachieving faster bonding and producing highly integrated circuitcomponents.

What is claimed is:
 1. A fine gold alloy wire for use in the bonding ofsemiconductor elements characterized by having high tensile strength andconsisting essentially from 0.0003 to less than 0.0010 wt% of at leastone rare earth element selected from the group consisting of La, Ce, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y, with the balancebeing Au and incidental impurities.
 2. The fine gold wire of claim 1wherein said rare earth element is selected from the group consisting ofLa, Ce, Pr, Nd, and Sm.
 3. A fine gold alloy wire for use in the bondingof semiconductor elements characterized by having high tensile strengthand consisting essentially of 0.0003 to less than 0.0010 wt% of at leastone rare earth element selected from the group consisting of La, Ce, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y, and 0.0001 to0.0060 wt% of of at least one element selected from the group consistingof Ge, Be and Ca, with the balance being Au and incidental impurities.4. The fine gold wire of claim 3 wherein said rare earth element isselected from the group consisting of La, Ce, Pr, Nd, and Sm.